1. Field of the Invention
The present invention relates to a system and method for detecting the interface between media having different indices of refraction, and more particularly, to a method of detecting the location, and hence the thicknesses and respective diameters, of the various layers of an optical fiber deposition tube or preform.
2. Discussion of Related Art
It is important to be able to identify the location of an interface or interfaces between media having different indices of refraction without contacting the media. For example, during the various stages of manufacturing an optical fiber preform (e.g., MCVD, tube collapse, etc.), it is advantageous to identify the location of the interfaces between the various layers, and particularly between a glass outer layer and an inner layer, for example, of gas, fluid, or vacuum, of a glass tube so that the concentricity and thickness of the layers can be determined as well as the overall diameter. Typically, a deposition tube, or preform, is a quartz tube used to form an optical fiber. It is important to have accurate measurements of the preform to predict the precise formation structure of the optical fiber drawn from the preform.
It is conventionally known that precise measurements may be made by hand using a contact measurement device, such as calipers. However, since the preform is often sealed at each end, a caliper may only be able to determine the outer diameter of the preform. Additionally, because the preform is often made of material such as quartz, contact measurements are undesirable as the material is susceptible to damage from the contact measurement device. Therefore, it is desirable to have a non-contact method of measuring inner and outer diameters of a preform.
There are various known non-contact techniques for measuring the thickness of a glass substrate, one requiring a laser beam and another requiring an ultrasonic pulse.
In the former technique, a laser beam is focused on the target surface using an objective lens coupled to a vibrating tuning fork producing lens vibration. As the vibrating lens moves the beam in and out of focus, the beam is continuously reflected off the target surface and back into the sensor where it is redirected by half-mirrors and converged on a pinhole aperture in front of a light receiving element. The beam only passes through the aperture when it is perfectly focused on the targets surface. At the true focal point, the peak quantity of light is detected by the light receiving element whereupon exact vertical position of the tuning fork is determined. Once the vertical position of the fork is known, the distance to the target""s surface is then calculated.
Another technique using a laser beam involves the use of triangulation to determine the distance from the laser to the laser spot produced on the surface of the object to be measured.
The ultrasonic technique is described in U.S. Pat. No. 5,585,563. This technique involves sending an ultrasonic pulse through a water transmission medium into the object to be measured and measuring the time difference between the returning echoes. The time difference is correlated to the thickness of the respective layers.
However, these techniques are both expensive and complicated. For example, the laser technique involving triangulation requires two separate units to measure the outer diameter of, e.g., a fiber optic rod increasing the complexity of the setup and the overall expense. Further, the ultrasonic technique requires the presence of water as a transmission medium between the detector and the object. Since the preform is very hot in most of the processes, it cannot come in contact with water.
An object of the present invention is to provide a relatively simple and inexpensive system and method for detecting the interface between media of different refractive indices and, hence, the thicknesses and respective diameters of the various layers without contacting the media.
A further object of the invention is to provide an easy method and device for determining the concentricity of the interfaces of media having different refractive indices, the ovality of the interfaces, and a ratio of maximum and minimum points of the media, by making measurements while the media is rotated.
The present invention is directed to a simplified and inexpensive system and method for detecting the interfaces between media having different indices of refraction. According to a preferred embodiment, the method involves placing a pattern (e.g., a white diagonal line on a black background) behind the media and observing the media from the front. The interfaces in the media, through a combination of refraction and reflection, reproduce an image of the pattern which indicates the apparent location of the interfaces. The actual location can then be calculated from the apparent location. On the other hand, the actual location can be seen, as opposed to calculated, using a telecentric lens. However, due to the small size and high priced nature of telecentric lenses, this option may be somewhat limited.
According to a preferred embodiment of the invention, a system for detecting the interface between mediums with different refractive indices includes an optical observation device positioned on one side of the medium to be measured, and a target having a contrasting pattern, positioned on the opposite side thereof. The optical observation device may then record an apparent location of interfaces from which the actual locations are subsequently calculated.
According to a preferred method of the present invention, a method for indicating the interface between media with dissimilar indices of refraction includes, placing a pattern on one side of a substantially transparent object to be measured, observing the pattern through the object to be measured form an opposite side thereof, and measuring the apparent location of the interface which differs from an actual location due to the viewed location being slightly in front of the actual location, and calculating the actual location based upon the measured apparent location.